High-speed imaging devices (high-speed video cameras) used for taking consecutive images of high-speed phenomena, such as explosions, destructions, combustions, collisions or discharges, for only a short period of time have been conventionally developed (for example, refer to Non-Patent Document 1 and other documents). Such high-speed imaging devices need to perform an ultrahigh-speed imaging operation that exceeds a level of approximately one million frames per second. Accordingly, they use solid-state image sensors capable of high-speed operations, which have special structures different from those of the conventional imaging sensors used in normal video cameras, digital cameras and similar devices.
One example of this type of solid-state image sensor is disclosed in Patent Document 1 and other documents. The devices disclosed in those documents are referred to as an “in-situ storage image sensor” (IS-CCD). An outline of this image sensor is as follows.
In this in-situ storage image sensor, a storage CCD for holding a specified number of record frames is provided for each photodiode functioning as a photo-receiver. This CCD is also used for transferring signals. During an imaging operation, pixel signals produced by photoelectric conversion by the photodiode are sequentially transferred to the storage CCD. After the imaging operation is completed, the pixel signals corresponding to the specified number of record frames stored in the storage CCD are collectively and sequentially read, and the images corresponding to the specified number of record frames are reproduced outside the image sensor. During the imaging operation, pixel signals exceeding the specified number of image frames are discarded from the oldest ones. Thus, the latest set of pixel signals corresponding to the specified number of frames are always held in the storage CCD. This means that, when the transfer of pixel signals to the storage CCD is suspended at the completion of the imaging operation, one can obtain the latest series of images ranging from the completion of the imaging operation back through a period of time corresponding to the specified number of record frames.
Thus, unlike general types of image sensors that require pixel signals to be extracted every time a set of pixel signals corresponding to one frame is obtained, the in-situ storage image sensor is characterized by its capability of acquiring a series of images at extremely high speeds over a plurality of frames. However, the number of storage CCDs that can be mounted on a single sensor is limited due to various factors, such as the limited area of a semiconductor chip and the restriction on power consumption. Accordingly, the number of frames available for the aforementioned high-speed imaging is limited. For example, the number of frames is approximately 100 in the case of the device disclosed in Non-Patent Document 1. This number of frames may suffice for some applications. However, for some types of phenomena or objects, the imaging operation does not require a very high speed (e.g. one million frames per second) but should desirably be continued for a longer period of time or over a larger number of frames. It is difficult for the aforementioned in-situ storage image sensor to meet the demands for such imaging.
Therefore, to support both an ultrahigh-speed imaging mode that has a limitation on the number of consecutive record frames and an imaging mode that is rather slow but has no limitation on the number of record frames, it is necessary to use both the previously described in-situ storage image sensor using CCDs and a commonly known image sensor, such as a CMOS image sensor. Such an imaging system will be expensive.
In the aforementioned high-speed imaging, it is important to perform the imaging in synchronization with the timing of the occurrence of a phenomenon under observation. This is achieved by a control process in which the imaging action is initiated or discontinued in response to an externally given trigger signal. To generate this trigger signal, the system normally includes another sensor, such as a contact sensor, position sensor, shock sensor or pressure sensor. However, in some situations, it is often difficult to obtain appropriate trigger signals by this method, as in the case where the sensor cannot be easily placed close to the object, where the imaging action must capture a spontaneous change in the object under observation, or where the target of imaging is a micro-sized object under a microscope.
To address these problems, an imaging system disclosed in Patent Document 2 uses a light-splitting means, such as a beam splitter or half mirror, provided behind an imaging lens. The light-splitting means separates incident light into two beams, which are respectively introduced into different imaging devices. One of these two imaging devices is dedicated to a monitoring function to detect a sudden change in the image. This imaging device generates a trigger signal, which is used to control the initiation and discontinuation of the storage of image signals produced by the other imaging device. This type of conventional imaging system requires optical parts to split incident light coming from the object of imaging into plural beams, and additionally needs more than one imaging devices (image sensors). Thus, the system will be large and complex, making it difficult to reduce the production cost. Decreasing the size and weight of the system is also difficult.